1. Field of the Invention
The present invention relates to a dielectric ceramic composition for temperature compensation which can be sintered in conjunction with an internal electrode made of a base metal at a low temperature under a reducing atmosphere, and which has a low dielectric constant and a high dielectric quality factor (Q), as well as to a multilayer ceramic chip capacitor and a ceramic electronic device each using the ceramic composition.
2. Description of the Related Art
In recent years, with the advent of the information age, there is an increasing requirement for electronic devices which increase processing speed and improve radio frequency characteristics. Multilayer ceramic chip capacitors used in, for example, high frequency circuit filters also require a low dielectric constant and a high dielectric quality factor. They are widely used as electronic devices that can provide a stable reference capacitance.
Recently, there are (Ca,Sr) (Zr,Ti)O3-based, MgO—TiO2-based, BaO—TiO2-based, and BaO—TiO2-REO-based dielectric ceramic compositions that can be used for multilayer ceramic chip capacitors. These dielectric ceramic compositions can be sintered at a high temperature of 1,200° C. or more. In this regard, in order to sinter dielectric layers made of such dielectric compositions in conjunction with internal electrodes, high melting point metals such as Palladium (Pd) and Platinum (Pt) must be used as the internal electrodes. However, Pd and Pt are more expensive and higher in resistivity, compared with base metals such as Ag and Cu. Due to higher resistivity, equivalent series resistance (ESR) and inductance (ESL) are increased at high frequency, resulting in a high dielectric loss and a low dielectric quality factor.
Therefore, in order to use base metals such as Cu as internal electrodes, the use of a dielectric ceramic composition which can be sintered at a low temperature is required. Internal electrodes made of base metals may be oxidized upon being sintered in air. Therefore, co-sintering of dielectric layers and internal electrodes must be effected in a reducing atmosphere.
Exemplary low temperature sinterable dielectric ceramic compositions are disclosed in Japanese Patent Application Laid-Open Publication No. 5-190020; U.S. Pat. Nos. 5,756,408 and 4,988,651; and Japanese Patent Application Laid-Open Publication No. 1-102806.
Japanese Patent Application Laid-Open Publication No. 5-190020 discloses a dielectric ceramic composition that comprises a major composition represented by the general formula: a (xBa-yCa-zSr)O-bSiO2-cZrO2—(d/2) Al2O3-eTiO2 (wherein 5 mol %≦a≦6 mol %, 10 mol %≦b≦70 mol %, 0 mol %≦c≦30 mol %, 0 mol %<d≦30 mol %, 0 mol %<e≦30 mol %, a+b+c+d+e=100 mol %; and x+y+z=1). The dielectric ceramic composition does not have non-reducibility and thus base metals such as Cu cannot be used as internal electrodes. Furthermore, due to the use of glass process of heating a raw material mixture to a high temperature of 1,600° C. or more, followed by quenching quickly, it is difficult not only to disperse glass powders, but also to control particle size thereof.
U.S. Pat. No. 5,756,408 discloses a glass ceramic sintered body that contains 30 to 70% by weight of (Ca,Sr)—Al—Zn—Si—O type composite oxide and 30 to 70% by weight of Ca oxide and Zr oxide or CaZrO3 as a filler. Unfortunately, the dielectric quality factor of the ceramic sintered body was not considered.
U.S. Pat. No. 4,988,651 discloses a dielectric ceramic composition represented by the general formula: xBaO-YSiO2-z{ZrO2(1−β)TiO2(β1)SnO2(β2)} (wherein x, y and z are weight percentages of respective components; x+y+z=100; β=β1+β2, 0≦β1, 0≦β2, 0.01≦β≦0.03). However, due to the use of TiO2, non-reducibility becomes poor.
Japanese Patent Application Laid-Open Publication No. 1-102806 discloses a dielectric ceramic composition represented by the general formula: [x(Ba(1−a)Sra)O-ySiO2-zZrO2]—Al2O3. The dielectric ceramic composition can be sintered at a low temperature under a non-reducing atmosphere. However, there are disadvantages in that a temperature characteristic of capacitance is ±100 (ppm/° C.)), a dielectric quality factor (Q) is 1,000, and an insulation resistance is very low in the level of 1012 Ωcm.
Low temperature sinterable dielectric ceramic compositions can also be used in multilayer ceramic circuit boards for electronic devices. This is because dielectric compositions for multilayer ceramic circuit boards must be sintered in conjunction with internal electrodes made of base metals with low melting point. The use of high melting point metals as internal electrodes undesirably leads to a high electric resistance. Multilayer ceramic circuit boards are used as substrates on which semiconductor elements or various electronic elements are mounted, to thereby miniaturize electronic devices.